1. Field of the Invention
The present invention concerns an integrated photonic device, in particular an integrated polarization splitting and rotating photonic device to be implemented in a wafer chip. This kind of device is conceived to receive through an optical fibre a light input, to manage the polarization thereof.
More specifically, the invention refers to a polarization and rotating photonic device, commonly referred as PSR device also in the following, comprising at least one first waveguide core, a second waveguide core, both said waveguide cores extending from an input section to an output section and being separated by a gap at least at the output section, a top cladding and a bottom cladding, said claddings extending along the whole optical guiding structure enclosing said waveguides there between, so as to form an optical guiding structure in a chip.
2. Description of the Prior Art
Polarization handling is one of the trickiest issues in Silicon Photonics. Due to the intrinsic high birefringence of the High index contrast waveguides, TE modes are propagated with a completely different phase and group velocity than TM modes, causing several impairments in the optical transmissions systems.
TE modes are commonly known by the skilled in the art as those modes having the electric field main component substantially parallel to the chip surface, while TM modes have the electric field main component substantially orthogonal to the chip surface.
Several attempts have been carried out to manage the intrinsic polarization dependence of High index contrast waveguides; to date, the most promising solution is to use a polarization diversity scheme as proposed in by Tymon Barwicz et al. “Polarization-transparent microphotonic devices in the strong confinement limit”—nature photonics |VOL 1|January 2007| www.nature.com/naturephotonics.
To implement a polarization diversity scheme a device called Polarization Splitter and Rotator (PSR) is needed, and his complementary device, known as Polarization Combiner, which is nothing else than a PSR with reversed input and output.
A PSR is, generally speaking, an integrated optical device comprising at least one input waveguide and at least two output waveguides, said device being adapted to receive an optical signal having a scrambled polarization—a mix of both TE and TM with random amplitudes and phases—and propagate them to a respective output port by splitting the TE and TM components while the TM component is also rotated into a TE mode due to a 3D symmetry-breaking with respect to the input waveguide.
Under this scheme, it is important to underline that the splitting and rotating processes do not necessarily take place in the aforementioned order i.e. a Polarization Rotator Splitter (PRS) could be envisaged instead of a PSR.
Examples of PRS are known from Daoxin Dai et al. “Novel concept for ultracompact polarization splitter-rotator based on silicon nanowires”—23 May 2011/Vol. 19, No. 11/OPTICS EXPRESS, and from Yang Yue et al. “Silicon-on-insulator polarization splitter using two horizontally slotted waveguides” OPTICS LETTERS/Vol. 35, No. 9/May 1, 2010.
In any case, due to the linear behaviour of the device, the final result is unchanged regardless of the processes order, namely the two input polarizations TE/TM will be converted into two TE modes propagating in the output waveguides.
The PSR or PRS disclosed in this prior art have two consecutive and well separated splitting and rotation sections (or rotation and splitting); this is not very length effective leading to a much longer device than actually needed, and constitutes a major disadvantage in a context where the close packet of the devices is an essential requirement for a cost effective product and loss reduction.
Further examples are known from Jing Zhang et al. “Silicon-Waveguide-Based Mode Evolution Polarization Rotator”, IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 16, NO. 1, JANUARY/FEBRUARY 2010; Wesley D. Sacher et al. “Si3N4-on-SOI Polarization Rotator-Splitter Based on TM0-TE1 Mode Conversion”; L. M. Augustin et al. “A Compact Integrated Polarization Splitter/Converter in InGaAsP-InP”, IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 19, NO. 17, Sep. 1, 2007; US 2014/0133796 A1; CN 203311029 U; and US 2014/0270622 A1.
This prior art drawback is inherent to the PSR (or PRS) schemes therein disclosed due to the waveguides arrangements therein adopted.
In Jing Zhang et al. an effective polarization rotation is achieved by exploiting an adiabatic principle and by breaking the symmetry with respect to the both axis. However, the fabrication process of the polarization rotator of Jing Zhang et al. requires two mask levels and a fine control of elements with a small feature size.
In optics, the adiabatic principle is defined by the adiabatic theorem, i.e. a physical system remains in its instantaneous eigenstate if a given perturbation is acting on it slowly enough and if there is a gap between the eigenvalue and the rest of the Hamiltonian's spectrum. Therefore, an adiabatic process is defined so as, when gradually changing conditions, to allow the system to adapt its configuration, hence the probability density is modified by the process. If the physical system starts in an eigenstate of the initial Hamiltonian, it will end in the corresponding eigenstate of the final Hamiltonian.
In conclusion, a slow change of a waveguide geometry over lengths greater than the wavelengths involved of many orders of magnitude, i.e. in this case μm vs. nanometers or hundreds of μm vs. few μm, leads to an adiabatic transformation of the light therein.
Another example of simple adiabatic rotator is disclosed in CN 103336330 A, exploiting a continuous 3D symmetry-breaking wherein the input section thereof is adapted to receive a scrambled TE/TM signal, and to propagate it to the rotator output section, which is adapted to support an orthogonal set of modes with respect to the input section. In other words, the rotator output consists of another TM/TE scrambled signal with TE and TM interchanged with respect to the input. However, the rotator output section cannot support the first two modes, i.e. a first and a second, having the same polarization.
In Tymon Barwicz et al., a complete PRS is described based on the rotator of Jing Zhang et al. and an adiabatic splitter the two sections being one after the other.
In the other prior examples the top-bottom symmetry-breaking is simplified with respect to the previous ones, since it is achieved with a uniform cladding or core layer and requires a single mask fabrication process at the expense of the device compactness.
On the other hand a slightly different physics is used for the conversion to a higher order mode.
Daoxin Dai et al. discloses an adiabatic polarization rotating waveguide wherein the inputted fundamental TM0 mode is converted into the higher order TE1 mode when the waveguide width is increased.
This effect is achieved by breaking the top-bottom symmetry of the Silicon waveguide by using a top cladding with a different material than the bottom one.
On the other hand the TE0 inputted mode is left unchanged and propagated to the TE0 mode at the rotator output.
At its input section the polarization rotator waveguide is single mode for both TE/TM polarization, while at the rotator output said waveguide supports at least two modes for the TE polarization, namely TE0 and TE1, and at least one mode for the TM polarization, namely TM0.
At the polarization rotator output wherein the TM0 has been TE1, a standard, non-adiabatic, directional coupler it is exploited to spatially separate the TE1 mode by coupling it into the TE0 mode of the adjacent waveguide.
Summarizing, in order for it to work, the Daoxin Dai et al. polarization rotator needs the silicon waveguide width to be increased until it becomes multi mode at least for the TE polarization, while at the same time the top-bottom symmetry is broken by the presence of a top-bottom cladding with a different refractive indexes.
Polarization splitting is then achieved with a standard directional coupler.
The device disclosed in Daoxin Dai et al. is a polarization rotator and splitter (PRS) wherein the polarization rotation and the splitting are achieved in two different, consecutives and well separated sections, the rotator section being adiabatic while the splitting one is non adiabatic standard directional coupler.
Yang Yue et al. discloses a polarization rotating device exploiting TM0-TE1 conversion wherein the top-bottom symmetry-breaking is achieved by an additional ridge layer having the same refractive index than the core one instead than exploiting two different claddings as in Daoxin Dai et al. Polarization splitting is then achieved with a standard, non adiabatic, directional coupler.
The device disclosed in Wesley D. Sacher et al. is a polarization rotator and splitter (PRS) wherein the polarization rotation and the splitting are achieved in two different, consecutives and well separated sections, the rotator section being adiabatic while the splitting one is non adiabatic standard directional coupler.
Wesley D. Sacher et al. discloses a polarization rotating device exploiting TM0-TE1 conversion wherein the top-bottom symmetry-breaking is achieved by an additional top layer having a lower refractive index than the core one instead than exploiting two different claddings as in Daoxin Dai et al. In the Wesley D. Sacher et al., polarization rotator the symmetry-breaking layer is made in SiN wherein the core waveguide is Silicon. Polarization splitting is then achieved with a standard adiabatic coupler.
The device disclosed in Wesley D. Sacher et al. is then a polarization rotator and splitter (PRS) wherein the polarization rotation and the splitting are achieved in two different, consecutives and well separated sections, both the rotator section and the splitter one being adiabatic.
In Wesley D. Sacher et al., the top-bottom symmetry-breaking layer does not extend through the entire device length but only across the polarization rotating section since it is not needed in the splitting section when done as per the Wesley D. Sacher et al.'s teachings.
In all the devices disclosed in the prior art, the polarization rotating section is separated by the polarization splitting one thereby leading to a relatively long devices having a length in the order of 400 μm or longer.
In the first section, namely the polarization rotating one, the higher order mode TE1 is excited starting from the TM0 inputted mode, in this sense the polarization is effectively rotated, but at the expense of exciting an higher order multi-lobed mode.
In the second section the TE1 mode is coupled to the TE0 mode of an adjacent waveguide thereby realizing the polarization splitting section which can be adiabatic as in the previous examples.
In Wesley D. Sacher et al., the top—bottom symmetry-breaking layer is tapered down before the start of the splitting section in order to complete transfer the TE1 mode generated in the first section to the underlying level before it can be coupled to the adjacent waveguide in the splitting section; this further increases the device length.
In prior art examples of a NON adiabatic PSR are also widely known.
For example, in “Fabrication tolerant polarization splitter and rotator based on a tapered directional coupler” by Yunhong Ding et al, 27 Aug. 2012/Vol. 20, No. 18/OPTICS EXPRESS, a PSR based on mode coupling instead than modes adiabatic evolution is reported.
The experimental signature of the prior art device being non adiabatic is for example its bandwidth which is much narrower than the devices herein disclosed and the TM/TE conversion rate length dependence which is sinusoidal with length in prior art while exhibits a saturated behavior in the devices herein disclosed.